On Wed, May 30, 2012 at 11:02:50AM -0400, Saggi Mizrahi wrote:
> If you don't want to add it in a parameter then you already suspect that you 
> are doing something wrong. Using a singleton instead of passing a parameter 
> doesn't make the dependency not there. It's just obscures it.

Right.  Passing around an internal 'singleton' (whether wrapped or not) is not
the correct course.

> I might not fully understand what you want to do but I think what you want is 
> to have MOM expect a certain interface.
> Then have an adapter class bridging the two interfaces.
> The pass the wrapped CIF to MOM.

It seems that your suggestion obscures the dependency at least as badly as a
singleton interface does.  Today clientIF is a singleton class in all but the
name.  It is basically a global container that stores global objects
(vmContainer, locks, etc) and global threads (irs, hostStatsThread, etc).  The
fact that a reference gets passed around doesn't really make it any cleaner.  We
can all agree that clientIF (in its current form) has overstayed its welcome.
Mark and Xu He Jie have some ideas on how to remove it and plan to tackle that
work.  That being said, MOM is an important feature for oVirt 3.1.  Do we really
want it to drop out of the release in order to focus on clientIF cleanups (which
certainly will not make 3.1)?

Using a singleton simply means that API objects can be instantiated without the
caller needing to be aware of clientIF at all.  In my opinion, this is a cleanup
because we are free to refactor clientIF without affecting the bindings or MOM
one bit.

> ----- Original Message -----
> > From: "Mark Wu" <wu...@linux.vnet.ibm.com>
> > To: vdsm-devel@lists.fedorahosted.org
> > Cc: "Dan Kenigsberg" <dan...@redhat.com>, "Saggi Mizrahi" 
> > <smizr...@redhat.com>, "Adam Litke" <a...@us.ibm.com>, "Ryan
> > Harper" <ry...@us.ibm.com>
> > Sent: Wednesday, May 30, 2012 10:49:29 AM
> > Subject: VDSM API/clientIF instance design issue
> > 
> > Hi Guys,
> > 
> > Recently,  I has been working on integrate MOM into VDSM.  MOM needs
> > to
> > use VDSM API to interact with it.  But currently, it requires the
> > instance of clientIF to use vdsm API.  Passing clientIF to MOM is not
> > a
> > good choice since it's a vdsm internal object.  So I try to remove
> > the
> > parameter 'cif' from the interface definition and change to access
> > the
> > globally unique  clientIF instance in API.py.
> > 
> > To get the instance of clientIF,  I add a decorator to clientIF to
> > change it into singleton. Actually, clientIF has been working as a
> > global single instance already.  We just don't have an interface to
> > get
> > it and so passing it as parameter instead.  I think using singleton
> > to
> > get the instance of clientIF is more clean.
> > 
> > Dan and Saggi already gave some comments in
> > http://gerrit.ovirt.org/#change,4839  Thanks for the reviewing!  But
> > I
> > think we need more discussion on it,  so I post it here because
> > gerrit
> > is not the appropriate to discuss a design issue.
> > 
> > Thanks !
> > Mark.
> > 
> > 
> 

-- 
Adam Litke <a...@us.ibm.com>
IBM Linux Technology Center
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